The 2000 Rosetta asteroid targets observational campaign: 140 Siwa and 4979 Otawara

Photometric and spectroscopic observations of 140 Siwa and 4979 Otawara, ta rgets of the Rosetta mission, have been carried out at the European Souther n Observatory (ESO, Chile). The complete lightcurves in V and R of both ast eroids and the Siwa spectrum in the range 0.95-2.52 mum have been obtained. We determined the precise synodic rotational period of 140 Siwa (18.495 h +/- 0.005 h) and we confirmed the synodic rotational period of 4979 Otawara (2.707 h +/- 0.005 h). The phase function has been obtained for both aster oids, allowing the determination of the H-V and G(V) parameters: for 4979 O tawara H-V = 14.62 mag, G(V) = 0.34; for 140 Siwa H-V = 8.34 mag, G(V) = 0. 17. The near infrared spectrum of Siwa does not show any spectral feature, which is consistent with a C/P type object.